The present invention relates generally to portable power tools and associated control systems therefor, and more particularly provides a unique stress-limiting flexible throttle lever structure for use in a gasoline engine driven power tool such as a flexible line trimmer, lawn edger, brush cutter or the like.
To control engine speed on a gasoline engine driven power tool of these and various other types, it is customary to provide a finger operated, pivotally mounted throttle lever which is connected via a suitable cable member to the engine's pivotally mounted carburetor throttle arm. By pivoting the throttle lever between its pivotal limit positions the carburetor may be operated at its idle position, its fully open position, or at any selected intermediate position therebetween.
As is well known, various dimensional inaccuracies, within a permissible tolerance range, are invariably present in the interconnecting cable and its associated throttle lever and carburetor throttle arm connecting structure. The result of these unavoidable dimensional inaccuracies is that the maximum stroke distance through which the carburetor throttle arm may be moved is not always precisely matched to the maximum stroke distance through which the throttle control lever may be moved. More specifically, in some instances the maximum stroke distance of the throttle lever is greater than that required to move the carburetor throttle arm through its maximum stroke.
When this particular dimensional combination arises, it is possible to move the throttle arm to its open limit position (in which the arm is typically moved against a motion limiting abutment) by moving the throttle control lever through only a portion of its available stroke distance. An attempt to force the throttle lever through the remainder of its stroke distance can accordingly impose undesirably high stress levels on the throttle lever, the cable and/or the carburetor throttle arm held in its open limit position. In turn, this can break or damage the throttle lever, the interconnecting cable, or the carburetor throttle arm.
A conventionally utilized solution to this dimensional variance problem has been to provide adjustment means for compensating for the mismatch between the stroke distances of the throttle control lever and carburetor throttle arm. Such adjustment means typically take the form of auxiliary adjustment structure incorporated into the overall throttle linkage structure and operative to vary the effective length of the interconnecting cable and/or increase or reduce the actual length of the throttle lever stroke distance.
The conventional need to incorporate such auxiliary adjustment structure gives rise to a variety of problems, limitations and disadvantages. For example, the need to utilize such auxiliary adjustment structure adds to both the material and assembly labor costs of the finished tool. There is additionally an added labor cost required to properly adjust the stroke compensating structure at the time of manufacture. Moreover, the use of such adjustment structure increases the difficulty encountered by the user of the tool in servicing the throttle control system and can lead to incorrect adjustment of the system by the user. Finally, the very existence of this adjustment structure gives rise to the need for periodic adjustment thereof by the user over the life of the system, together with similar adjustment when any component of the system is serviced or replaced.
It is accordingly an object of the present invention to provide improved throttle control apparatus which eliminates or minimizes above-mentioned and other problems, limitations and disadvantages typically associated with conventional adjustable throttle lever and associated linkage apparatus.